Re: [PATCH v2 2/3] perf/ibs: Fix interface via core pmu events

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Mar 9, 2023 at 2:12 AM Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>
> Although, IBS pmu can be invoked via it's own interface, indirect
> IBS invocation via core pmu event is also supported with fixed set
> of events: cpu-cycles:p, r076:p (same as cpu-cycles:p) and r0C1:p
> (micro-ops) for user convenience.
>
> This indirect IBS invocation is broken since commit 66d258c5b048
> ("perf/core: Optimize perf_init_event()"), which added RAW pmu
> under pmu_idr list and thus if event_init() fails with RAW pmu,
> it started returning error instead of trying other pmus.
>
> Fix it by trying to open event on all pmus if event_init() on user
> requested pmu returns -ESRCH.
>
> Without patch:
>   $ sudo ./perf record -C 0 -e r076:p -- sleep 1
>   Error:
>   The r076:p event is not supported.
>
> With patch:
>   $ sudo ./perf record -C 0 -e r076:p -- sleep 1
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 0.341 MB perf.data (37 samples) ]
>
> Note that there is no notion of forward pmu mapping. i.e. kernel doesn't
> know which specific pmu(or a set of pmus) the event should be forwarded
> to. As of now, only AMD core pmu forwards a set of events to IBS pmu
> when precise_ip attribute is set and thus trying with all pmus works.
> But if more pmus starts returning -ESRCH, some sort of forward pmu
> mapping needs to be introduced through which the event can directly
> get forwarded to only mapped pmus. Otherwise, trying all pmus can
> inadvertently open event on wrong pmu.
>
> Fixes: 66d258c5b048 ("perf/core: Optimize perf_init_event()")
> Reported-by: Stephane Eranian <eranian@google.com>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
> ---
>  arch/x86/events/amd/core.c | 11 ++++++++---
>  kernel/events/core.c       | 10 +++++++++-
>  2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 8c45b198b62f..81d67b899371 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -371,10 +371,15 @@ static inline int amd_has_nb(struct cpu_hw_events *cpuc)
>  static int amd_pmu_hw_config(struct perf_event *event)
>  {
>         int ret;
> +       u64 dummy;
>
> -       /* pass precise event sampling to ibs: */
> -       if (event->attr.precise_ip && get_ibs_caps())
> -               return -ENOENT;
> +       if (event->attr.precise_ip) {
> +               /* pass precise event sampling to ibs by returning -ESRCH */
> +               if (get_ibs_caps() && !ibs_core_pmu_event(event, &dummy))
> +                       return -ESRCH;
> +               else
> +                       return -ENOENT;
> +       }
>
>         if (has_branch_stack(event) && !x86_pmu.lbr_nr)
>                 return -EOPNOTSUPP;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f79fd8b87f75..e990c71ba34a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -11639,18 +11639,26 @@ static struct pmu *perf_init_event(struct perf_event *event)
>                         goto again;
>                 }
>
> +               /*
> +                * pmu->event_init() should return -ESRCH only when it
> +                * wants to forward the event to other pmu.
> +                */

Can we add this to the comment in the struct pmu?  There is a
description already for other error codes.

Otherwise looks good.

Thanks,
Namhyung


> +               if (ret == -ESRCH)
> +                       goto try_all;
> +
>                 if (ret)
>                         pmu = ERR_PTR(ret);
>
>                 goto unlock;
>         }
>
> +try_all:
>         list_for_each_entry_rcu(pmu, &pmus, entry, lockdep_is_held(&pmus_srcu)) {
>                 ret = perf_try_init_event(pmu, event);
>                 if (!ret)
>                         goto unlock;
>
> -               if (ret != -ENOENT) {
> +               if (ret != -ENOENT && ret != -ESRCH) {
>                         pmu = ERR_PTR(ret);
>                         goto unlock;
>                 }
> --
> 2.39.2
>